The present invention relates to multichannel cochlear prosthesis, and more particularly to a multichannel cochlear prosthesis that offers selectable and flexible control of a full spectrum of stimulus waveforms that are applied to the cochlea through each of the channels of the system.
Cochlear prostheses produce sensations of sound in deaf patients by direct electrical stimulation of the auditory nerve. In modern, multichannel cochlear prostheses, several different sites are stimulated at various distances along the cochlea to evoke the different pitches of sound perception that are normally encoded by nerve activity originating from the respective sites. The patterns of electrical stimulation are derived from acoustic signals picked up by a microphone and transformed by a so-called speech processor that is programmed to meet the particular requirements of each patient. Several different schemes for processing the acoustic signal and transforming it into electrical stimuli have been developed and are well-described in the scientific literature and various patents. These schemes can generally be divided into two basic types on the basis of the waveforms of the electrical stimuli:
i) Analog waveforms, which are essentially filtered versions of the continuous acoustic waveform, usually involving dynamic range compression, bandpass filtering and scaling to the stimulus current ranges that evoke a satisfactory range of auditory sensations from threshold of perception to maximal comfortable loudness. This produces a rich but poorly controlled set of resultant waveforms because, inter alia, the waveforms are susceptible to degradation by an electrode array that does not have fully isolated channels. PA1 ii) Biphasic pulses, or, in more general terms, multiphasic pulses. Biphasic pulses consist of a single cycle of a pulsed wave in which current flows in one direction at a specified magnitude and for a specified brief period of time and is followed immediately by an opposite direction of current of a similar magnitude and duration. Multiphasic pulses comprise a plurality of pulsed waves in which current flows first in one direction, then in another direction, and so on, as required, at specified magnitudes and brief periods of time, in such a way that the charge associated with the total of all the plural pulses is balanced, whereby the net electrical charge delivered to the tissue over one multiphasic cycle is zero. These biphasic or multiphasic pulses are most often delivered in sequence to various sites, with the instantaneous magnitude at each site proportional to some measure of the amount of energy present in a particular frequency band of the acoustic waveform. The result is an impoverished (in terms of bandwidth and phase) but precisely controlled set of stimulus waveforms.
Both of the above stimulus waveform types--analog and biphasic/multiphasic--have been selected because they are relatively easy to produce and modulate electronically for real-time encoding of speech. Further, the waveform shape can readily be controlled so as to guarantee a charge-balanced alternating current at the electrodes, avoiding net direct current that is known to cause electrolytic damage to both electrodes and body tissues.
For purposes of the present application, the spatiotemporal manner in which either of the above two types of stimulus waveforms are applied to the cochlea of a patient is referred to as a "speech processing strategy." The spatial application of the stimulus waveforms is controlled by the type of electrode coupling, e.g., bipolar or monopolar, through which the stimuli are applied to various locations along the inside of the scala tympani duct of the spiral-shaped cochlea. The temporal application of the stimulation waveforms is derived by the timing of the stimuli. Traditionally, speech processing strategies have thus been classified as either: (1) a simultaneous strategy, or (2) a non-simultaneous, or sequential, strategy. Analog waveforms have traditionally been applied as part of a simultaneous strategy, relying on the more highly focused stimulation provided by bipolar coupling to produce an electrical pattern that will yield speech intelligibility. Short, non-simultaneous, biphasic or multiphasic pulses, on the other hand, are usually applied as part of a sequential speech processing strategy, relying on the highly precise sequence of stimulation pulses through monopolar coupling to produce the electrical pattern that yields speech intelligibility. In a typical sequential, or non-simultaneous, strategy, short biphasic pulses are applied in rapid succession (with little or no time overlap) in a specified pattern to each of multiple channels.
Not all patients benefit from the same speech processing strategy. That is, the complex biophysical phenomena associated with the electrical excitation of neurons and psychophysical phenomena regarding the interpretation of neural activity by the auditory nervous system suggest that the quality and intelligibility of speech percepts evoked by a cochlear prosthesis may be improved in a given patient by more specific manipulations of the electrical stimulus waveforms tailored to that patient.
When a cochlear prosthesis is first provided to a patient, including implanting any implantable components of the prosthesis into the patient (which implantable components typically include at least an electrode and a stimulator, and may also include, for fully implantable systems, a speech processor, a microphone and/or a rechargeable power source), it is necessary to initially "fit" or "adjust" the prosthesis. As used herein, it should be noted that the terms "fit", "adjust", "fitting" or "adjusting" relate to making electronic or software-programming changes to the prosthesis, as opposed to making physical or hardware changes, for the purpose of making the prosthesis better perform its intended function of helping the deaf patient to sense sound. Where more than one speech processing strategy is available within the prosthesis, selecting a speech processing strategy that is best suited for a particular patient thus becomes a key part of the "fitting" or "adjusting" process. Moreover, it should be recognized that the fitting or adjusting process will typically continue to occur at regular or as-needed maintenance/check-up intervals after the initial fitting. Thus, it is seen that where more than one speech processing strategy is available within the prosthesis, the patient, e.g., through the assistance of his or her physician or audiologist, may receive the benefit of, or at least try out during a trial period, one or more different speech processing strategies than was used initially.
Disadvantageously, not all cochlear prosthesis are capable of providing more than one speech processing strategy, or of providing both simultaneous and non-simultaneous speech processing strategies. Rather, most commercially-available cochlear prosthesis provide just one type of strategy (simultaneous or non-simultaneous), although some may offer, for example, multiple non-simultaneous strategies. Hence, with these one-type-strategy devices, the patient, and/or the physician/audiologist fitting the patient, has no, or only a very limited, ability to select a suitable speech processing strategy that is most effective for that patient. The Clarions cochlear stimulator, available from Advanced Bionics Corporation, of Sylmar California, is the only known commercially-available cochlear stimulator that allows the user to select either a simultaneous speech processing strategy or a non-simultaneous speech processing strategy as the technique for encoding the acoustic input signal into an electrical pattern that yields speech intelligibility. The Clarion.RTM. stimulator is described, inter alia, in U.S. Pat. No 5,603,726, which patent is incorporated herein by reference.
In U.S. Pat. No 5,626,629, incorporated herein by reference, there is disclosed one approach for fitting a Clarion.RTM.-type cochlear prosthesis, i.e., a multichannel cochlear prosthesis that offers more than one speech processing strategy, to a patient. While the techniques and tools taught in the '629 patent are very usefull and effective for many, if not most, fitting issues, an effective way to select an appropriate speech processing strategy for a particular patient is not addressed. Rather, the '629 patent assumes that one of a plurality of speech processing strategies will be selected, and that the fitting process will go forward based on that selected strategy. Hence, there remains a need for assisting the clinician, audiologist and/or physician in identifying which one(s) of several possible speech processing strategies are best suited for use within a given patient's multichannel cochlear prosthesis.
In U.S. Pat. No. 5,601,617, also incorporated herein by reference, there is disclosed a straightforward and understandable way of defining complex stimulation waveforms--both simultaneous and non-simultaneous waveforms--for use within a multichannel cochlear prosthesis, e.g., a Clarion.RTM. cochlear prosthesis. Disadvantageously, while the technique disclosed in the '617 patent is highly suitable for defining complex waveforms for use within a multichannel cochlear prosthesis, there remains a need for assisting the clinician, audiologist and/or physician in discovering which one(s) of several possible speech processing strategies--ranging from simultaneous analog strategies on one end of the spectrum to non-simultaneous pulsatile strategies on the other end of the spectrum--are best suited for use within the patient's multichannel cochlear prosthesis.
Where only sequential pulsatile speech processing strategies are used, it is known to specify the number of maxima N out of M possible sequential pulsatile channels that are to be used as part of the speech processing strategy. For example, where M sequential pulsatile channels are available, any number N of these M pulsatile channels, where N is thus an integer from 1 to M, may be selected as the number of maxima. Once the number of maxima is set by the selection process, the speech processing strategy then applies a suitable algorithm to determine which N of the M channels are to be stimulated per data frame. Typically, the N channels with the greatest input levels are selected for stimulation. See, e.g., Wilson, et al., "Comparative Studies of Speech Processing Strategies For Cochlear Implants.revreaction.. 90th Annual Meeting of American Laryngological, Rhinological and Otological Society, Inc. (Denver, Colo. Apr. 1988). See also, U.S. Pat. Nos. 5,597,380; 5,271,397; and 5,095,904. Disadvantageously, while this "NofM" approach provides some selection ability with respect to sequential pulsatile channels, there remains a need to be able to use it in combination with simultaneous channels.
The present invention advantageously addresses the above and other needs.